The building materials industry is favoring the use of water-based adhesives for adhering floor coverings, such as carpet, tile, wood, epoxy, etc., onto the top surface of concrete floor slabs. This switch is partly caused by a need to avoid adhesives that are based on volatile organic compounds (VOC) and their attendant risks to health. Along with the benefits provided by water-based adhesives, there is both a short term problem and a long term problem.
A short-term problem arises due to the fact that a substantial amount of moisture evaporates from the concrete floor slab after it is cast. This prolongs the time before the water-based adhesive can be applied onto the surface of the floor slab. It is recognized in the flooring industry that water vapor emissions should be below 5 pounds/1000 square feet/24 hours, and preferably below 3 pounds/1000 square feet/24 hours, before a water-based adhesive coating is applied. For concrete typically used in floor slabs, it will usually take 60–90 days before this level of vapor transmission is achieved.
A long-term problem arises due to the fact that cured concrete is not impervious to water vapor transmission. Consequently, a coating of water-based adhesive (e.g., rubber latex, vinyl acetate, polyacrylate, etc.) applied onto the top of the floor slab can fail over time if the environment below the slab becomes saturated with moisture. Such saturation may occur due to water table fluctuations or other environmental causes.
To resolve the long term problem, most industry experts (such as the American Concrete Institute) have begun to recommend that floor slabs be placed directly on top of a vapor barrier membrane. However, current practice is simply to place concrete onto a liner sheet (e.g., polyethylene). This liner sheet does not bond with the concrete, and has a potential failure mode whereby water or vapor passes between adjacent liner sheets at the overlap, or through punctures in the liner sheets. This water or vapor can then spread out between the liner sheet and the concrete over a wide area, especially if local settlement of the subgrade occurs. In providing an advantage over simple liner sheets (e.g, vinyl sheet), self-adhered vapor barrier membranes have a waterproofing adhesive layer that is operative to bond with concrete that is wet cast against the adhesive layer, so as to form a continuous waterproofing bond.
Thus, while placement of concrete directly upon a self-adhering vapor barrier membrane can solve the long-term problem of water leakage, the present inventors realized that the short-term problem may be exacerbated by the use of membranes. The placement of a vapor barrier membrane between the sub-grade environment and the concrete floor slab means that excess water in the concrete will not be able to escape into the sub-grade environment; but, rather, will be forced to evaporate out of the top surface of the concrete floor slab as it dries. This can result in longer waiting periods before a water-based adhesive coating can be applied onto the upward facing floor slab surface, and may delay completion of the project. Moreover, the fact that the slab dries from only one surface rather than from both the bottom and top surfaces creates a moisture gradient within the concrete slab that may cause the slab to curl. This curling phenomenon increases the risk of cracking and can lead to serviceability problems when the floor is no longer flat.
Accordingly, the present inventors believe that a novel concrete flooring system and method for making the flooring are needed.